Motor vehicles are equipped with safety belts and airbags for passenger safety. A wide variety of occupant sensors provide input to the Airbag Control Module (ACM) relating to the presence or absence of occupants in the several seats served by these safety devices. Prior patents show ultrasound and IR sensors, capacitance sensors, floor mat weight sensors and seat sensors. The latter include capacitance and weight sensors, to name two principal types. The trend in recent years is to employ occupant weight sensors in association with the automotive seat that provide weight data to the ACM, to determine whether or not to deploy the airbag, dependent on occupant seating conditions.
Further, in the case of smart airbags, the rate of inflation, the extent of inflation (gas quantity), and the timing of inflation may be controlled depending on the sensed weight of the seat occupant. For seat belts, the tension of the seat belt of a particular seat can be adjusted, e.g., by taking up slack prior to crash event, by the firing of seatbelt squibs upon sensing of an imminent crash combined with signal data indicative of the presence of an occupant in the selected seat.
Occupant weight sensors that are mounted in association with an automotive seat include a wide variety of types, design configurations and mounting locations. One class of sensors includes full or partial mat type sensors that employ pressure contacts; these mats can be integrated into the seat cushion and/or the back cushion.
Another significant class is that of load cells, also know as seat weight sensors that employ thick or thin film strain gauges that are screen printed on a steel substrate to measure the weight of a seated occupant in an automotive seat. The sensor substrate is typically a steel plate on the order of a few millimeters in thickness to provide a selected deflection before yielding.
It is essential to keep the deflection of the sensor substrate in the elastic region of the sensor substrate material's yield strength over the typical range of automotive use, including environment temperature variation, which is on the order of −40 to +90° C. Deformation beyond the elastic limit of the substrate under load can result in false or distorted sensor output and ultimately to mechanical or electrical failure of the sensors, including physical separation of the sensors from the substrate. Of course, erroneous or false readings or failure will lead to improper inputs to the airbag controller, possibly resulting in deployment when the seat is empty, or non-deployment when it is occupied, both unwanted results.
One largely ignored problem is that seat and occupant loads are not purely vertical. Forces and moments from all directions come to the sensor. Current sensor designs have components and mechanisms which filter out all loads except pure vertical loads before they come to the sensor. This is expensive, adds weight to the seat and also occupies precious packaging space in the seat. If the moments are not filtered out, the sensor substrate can quickly fail under a twist about the longitudinal or transverse axis of the sensor—without an overload stop for moments. The Z axis is the vertical axis through the stud and is also called the load axis. The rotation of the load stud around the X and Y axes gives rise to moments, Mx and My, respectively. These moments can disproportionately affect sensor outputs. Even small yields can lead to false, erroneous readings or failure. Significantly, typical non-load-stop design sensors can be strong in the Z axis, but weak under an Mx or My moment.
The strain gauges are distributed on the sensor substrate in a wide variety of geometric configurations, typically symmetrical with respect to the longitudinal and lateral axes of the sensor substrate plate, and are connected in full Wheatstone bridge configuration to measure the strain caused by weight-induced bending, primarily in the vertical direction. A discussion of the complex processes involved in the construction of thick film strain gauges is found in DE 199 48 045 A1 (1998; Takata Corp., Shiga, JP; H. Aoki), the thin film strain gauges of which are disclosed on the longitudinal axis of the sensor substrate at the juncture of thinned-down regions flanking a center lug and intermediate of the two end lugs.
The load cells are used in an array, typically distributed under the four corners of the cushion pan or the coordinate location on or in association with a seat track, or other seat support structural component(s). The strain gauge signal outputs are analyzed by a microchip in accord with well-understood algorithms to provide appropriate input to vehicle restraint system(s), such as smart airbag modules, seat belts, pretensioners, warning systems and the like.
Examples of various types of sensors, mounting locations and circuits that are shown in the art include: U.S. Pat. No. 6,161,891 (1999; CTS Corp.; Blakesley) which employs a dogbone design sensor substrate with resistors on the juncture of the necked-down region between two end lugs; U.S. Pat. No. 5,991,676 (1996; Breed Automotive Technology, Inc.; Pololoff et al.) which employs circular variable resistance force sensors mounted around the four seat corner mounting bolts; U.S. Pat. No. 6,069,325 (1998; Takata Corp; Aoki) which employs seat mounts having pivoting lever arms associated with load sensors; U.S. Pat. No. 6,231,076 B1 (1999; CTS Corporation; Blakesley et al.) which employs stepped sensor substrates mounted diagonally between the sides of an angle bracket between the seat pan and seat slide rail; U.S. Pat. No. 6,201,480 B1 (1998; Takata Corporation; Aoki) which shows load sensors on the four corners of a seat and a variety of Wheatstone bridge circuit configurations; and U.S. Pat. No. 5,810,392 (1997; Breed Automotive Technology; Gagnon) which s hows compression type sensors located in the seat cushion between a rigid member and the seat pan, the seat having an internal spring grid to bear a portion of the occupant weight.
The thickness dimension of the sensor substrate, being relatively thin, limits the load capacity of the sensor substrate. That is, if the sensor substrate is too thick, it will not measurably deflect under load, the deflection is difficult to measure, or the measurements become inconsistent and subject to thermal condition interference (thermal noise). To prevent the sensor substrate from failing load limit stops are employed. These stops, typically provided by support brackets, limit the load stud travel in the up and down direction and prevent excessive rotation (moments about longitudinal and lateral axes).
To obtain greater range of gauge readings, the sensor substrate can be made thinner, but the serious disadvantage is that the range of deflection before yield is reduced, and the load capacity of the load cell is reduced. In addition, thinner substrates are more prone to distortions and failure under Mx and My torsional forces. Since the load cells are expensive to produce, and generally are currently assembled and mounted to the seat during seat construction, adding more load cells is not a realistic alternative. The current load cells require very high precision, the deflection travel amounting typically to ±0.05 mm. This precision is difficult to achieve, particularly over large production runs, and even more difficult to maintain over time within the environmental conditions of automotive use, particularly vibration, large thermal range, and passenger misuse. Further, failed load cells are difficult and expensive to repair, as they generally require substantial, if not entire, disassembly of the seat.
Two recent patents, Aoki U.S. Pat. No. 6,323,443 and Aoki U.S. Pat. No. 6,323,444, are directed to load sensors in which concave springs are used as spacers on the load stud. In the '443 patent, stops are provided for vertical Fz load and transverse Fy forces in several designs. In one design (FIG. 3) where the substrate is supported at its ends by solid bolts and the load is borne in the centrally located load stud, the load bolt has minimal clearance with respect to the substrate. An upward Fz stop is provided by a large under-hanging flange of a retainer collar. A larger gap between a shoulder on the collar and a base plate (not the substrate) provides an Fy stop. In a second, cantilevered design (FIG. 4) where the substrate is supported only at one end, a non-load bearing rod projects through a restriction hole in the base plate and terminates in a flange. The flange and hole in the base plate provide de-centralized up Fz and lateral Fy stops. A down Fz stop is provided by the clearance between a substrate-securing bolt and the underlying base plate; again this stop is not centralized. The concave springs do not bear on the substrate. Significantly, no mention is made in the '443 patent regarding any mechanical stop or limit features to address the serious Mx/My problems.
The Aoki '444 patent is an application of the principles of the '443 patent, disclosing complex cantilevered designs which are described as applying moments to the outboard, unsupported ends of the sensor substrate, which is securely bolted to a base plate only in the center. Mx/My as applied to a central load stud and centralized stops are not disclosed. This patent states that all loads are absorbed by the pivot mechanism, and only vertical loads are allowed to reach the middle of the sensor substrate. Accordingly, there is a current unmet need in the art to provide load cell type seat sensors that, inter alia: have moment stops for Mx and My; have centralized stops for Fz, both up and down, for Fx and Fy, both directions, and for Mx and My moments; are modular; can be pre-assembled and mounted as a drop-in unit at the time of seat construction; are independent, as to their performance, of the precision of manufacture of seat components; are modular for swap-out repair; maintain excellent, repeatable response with precision over full load range; are less susceptible to automotive environmental noise (e.g., vibration and thermal noise); are easy to manufacture with less rigorous tolerances; have universal basic sub-assemblies that can accept a range of selected additional parts permitting them to be tailored to a wide range of seat designs; are robust; and are low cost to manufacture, install and repair.